As is known, the portion of the foot that has the greatest perspiration effect is the sole.
Therefore, the region of the shoe where moisture produced by perspiration can accumulate most is the interface between the sole of the foot and the sole of the shoe.
Here, the sweat that is produced saturates the air with moisture and mostly condenses, stagnating on the footbed.
Only a marginal fraction of the moisture produced by perspiration diffuses to the sides of the upper and exits from them if they are vapor-permeable.
This effect of stagnation of perspiration in the plantar region is particularly conspicuous in shoes that have a rubber sole; in such cases vapor permeation through the sole is in fact prevented by its total waterproofness.
As is known, the stagnation of sweat in the plantar region produces in the user of the shoe a feeling of discomfort and constitutes a preferential location for the growth of bacterial cultures, which notoriously cause bad odors.
The need to obviate the stagnation of moisture produced by perspiration at the plantar region of shoes is therefore a commonly felt need.
A first attempt to meet this need consists of the solution disclosed in Italian patent no. 1232798.
The teaching contained in said patent consists in dividing the rubber sole into two layers, the lower of which has through micro-holes, and in interposing between them a semipermeable membrane that is joined perimetrically to the two layers, in order to avoid infiltrations of water and thus obtain a sole that is impermeable to water in the liquid state and is permeable to water vapor.
For the sake of simplicity, hereinafter an element with the property of being impermeable to water in the liquid state and permeable to water vapor is indicated as waterproof and vapor-permeable.
The semipermeable membranes that the inventor of Italian patent no. 1232798 teaches to use are for example of the type described in U.S. Pat. Nos. 4,187,390 and 4,194,041 in the name of W. L. Gore or U.S. Pat. No. 6,228,477 in the name of BHA Technologies.
These membranes are provided by means of thin films of expanded polytetrafluoroethylene, e-PTFE, with thicknesses that generally vary from 15 to 70 microns, and are waterproof and vapor-permeable.
Their microstructure is characterized by the presence of dense areas, known as nodes, which are interconnected by elongated filaments, known as fibrils.
These semipermeable membranes, initially conceived for the military sector, have been developed and used in the field of clothing and shoes to avoid the accumulation of water vapor caused by perspiration in items of clothing and provide shoes with uppers with waterproof and vapor-permeable linings.
Since the market of the clothing and shoes sectors has always required soft and comfortable items, in the described applications there is a strong need to make sure that the membrane, understood as a functional layer, does not compromise these characteristics.
This requirement has developed into an actual technical prejudice, which has entailed the use of membranes provided in low thicknesses in order to be laminated with support and/or aesthetic finishing materials, such as fabric or leather, so as to obtain finished laminates that have enhanced characteristics of flexibility, ease of bending, softness, surface slipperiness, compressibility and extensibility and low weight per unit surface.
However, the films that provide these membranes have poor characteristics of mechanical strength, indeed due to their low thickness.
Indeed, it should be noted that the value of resistance of the laminate derives mainly from the characteristics of the layer of fabric or of the support with which the membrane is coupled.
In particular, available films of polymeric material, used to provide said membranes as mentioned, have thicknesses generally from 15 to 70 microns, which give them limited penetration resistance, i.e., less than 5 N. The expression “penetration resistance” is used to reference the characteristic defined by a measurement performed according to the method presented in the ISO 20344-2004 standard in chapter 5.8.2, “Determination of the penetration resistance of the sole” related to safety shoes.
This limited mechanical resistance to penetration has led the inventor of said Italian patent no. 1232798 to prevent contact of the membrane with foreign objects by limiting the diameter of the holes of the sole which the membrane faces.
However, this solution has proved to limit drastically the area of the sole assigned to vapor permeation.
The teaching contained in Italian patent no. 1282196 by the same Applicant proposes a solution that is aimed at overcoming these drawbacks.
This patent discloses a shoe with a sole made of elastomer which is provided with through holes and comprises a midsole that comprises a waterproof and vapor-permeable membrane superimposed on a protective layer, preferably made of felt, which is treated so as to be water-repellent.
Since the protective layer is not waterproof, it is not possible to perform a direct seal of the midsole with the sole, but a perimetric waterproof element is used which provides a sealing bridge between the waterproof and vapor-permeable membrane and the sole.
This invention, while allowing effective protection of the membrane against the penetration of external objects, has some aspects that can be improved.
In particular, the connection of the protective layer to the membrane, in order to allow an effective protection thereof, must occur by way of their intimate adhesion.
For this purpose, adhesives and glues are used which, while not covering all of the surface of the membrane, delimit the vapor-permeable portion of the part that they affect.
Moreover, the protective layer itself, though being vapor-permeable, is an obstacle to the passage of water vapor from the inside to the outside of the shoe through the midsole.
This drawback increases when, during use of the shoe, the protective layer progressively loses its characteristics of water-repellence and therefore tends to become impregnated with water and/or mud or other types of dirt, absorbed through the holes of the tread, compromising its vapor permeability.
A drawback of both of said proposed solutions consists in that during use the cyclic flexing and traction to which the midsole is subjected during walking tends to cause progressive wear and tear of the membrane, thus causing a loss of waterproofness at the sole.
In order to obviate this drawback, Italian patent no. 1282196 teaches to provide soles that have small holes, for example with dimensions of 1.5-2.0 mm, that are sufficiently spaced so as to not compromise their structural stiffness.
In this manner, the sole supports the midsole rigidly enough to contrast the tear of the membrane.
However, the portion of sole that can be crossed by water vapor is limited indeed by the rarity and narrowness of the holes with which it is provided.
A further solution, aimed at proposing a compromise between the need to contrast the tear of the membrane and the need to allow effective disposal of water vapor produced by perspiration through the sole, is described in Italian patent no. 1334928 by the same Applicant.
This patent discloses a sole having a structure that comprises a supporting layer which, at least in one macroportion, is made of mesh, felt or other diffusely perforated material.
A membrane made of material that is waterproof and permeable to water vapor is associated in an upward region with the supporting layer so as to cover at least the macroportion thereof.
Moreover, a sole made of polymeric material, with at least one macrohole that passes through at the macroportion, is joined so as to form a seal to the membrane and to the supporting layer at the perimeter of the macroportion.
This patent also contains the teaching of structuring the membrane by reinforcing it with protective layers and tear-resistant layers, made for example of nylon mesh intimately associated with the waterproof and vapor-permeable membrane.
It is also known to reinforce the membrane with high-strength layers made for example of Kevlar fabric.
Accordingly, the presence of the macrohole allows to define a large surface of the membrane that is adapted for heat exchange and for the exchange of water vapor with the outside of the shoe; simultaneously, the loss of structural stiffness of the sole is compensated, to contrast the tear of the waterproof and vapor-permeable membrane, by the supporting layer.
The advantage provided by having a large portion of sole free for vapor permeation is reduced, however, by the obstacle to the passage of water vapor that is constituted by the presence of the several layers that compose the supporting layer.
A further solution is described in U.S. Pat. No. 6,508,015 by Rauch Max.
This patent discloses a sole provided by means of a two-layer structure, respectively an elastic upper layer, which is permeable to water vapor, and a lower layer, which covers less than 70% of the upper layer and also acts as support and tread.
In this case, the vapor-permeable activity of the sole is ensured by the microporous structure of the upper layer and by the open shape of the lower layer.
The microporous structure of the upper layer is not waterproof and is made for example of sintered plastic material or by means of woven or non-woven structures made of synthetic material.
Although this solution proposes a large vapor permeation area of the sole, it does not allow to obtain a sole that is effectively waterproof, not even by adopting the refinements suggested in the cited patent, i.e., by treating the microporous material of the upper layer so as to make it hydrophobic or by providing a further upper layer formed by a thin waterproof membrane.
It has in fact been observed that the hydrophobic treatment of the sintered material does not make the upper layer sufficiently or stably waterproof.
Besides, the association of a waterproof membrane to the upper layer has the same drawbacks described as regards the solution that has a membrane supported by a protective layer.
Moreover, a sole provided according to the teaching of this patent, by having the microporous upper layer left extensively exposed by the lower supporting layer, tends to have a progressive degeneration of the upper layer upon contact with water, which is absorbed by capillary action together with the dirt that said water can convey.
The upper layer, once it is fully impregnated with dirt, is no longer vapor-permeable and can rot.
This drawback becomes all the more evident as the porosity of the material that constitutes the upper layer increases.